The higher the iLVL, the higher the stat cap on materia.
This said, STILL, some materia caps very low in some spots, and higher in others.
For example, Dex and Str cap much higher on gloves, because they are associated with the stat, while int and mnd cap higher on head pieces, vit on body, etc.
For straight stats, they seem to cap much lower than parry / crit / etc. Its because stats are very very powerful now, and a +2 dex is a huge difference.